This invention relates generally to methods and apparatus for CT imaging and other radiation imaging systems and, more particularly, to utilizing a method to minimize motion artifacts caused by cardiac motion.
In at least some xe2x80x9ccomputed tomographyxe2x80x9d (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, generally referred to as an xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through an object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at a detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged, so the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator adjacent the collimator, and photodetectors adjacent to the scintillator. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield units,xe2x80x9d which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time required for multiple slices, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved in the z-axis synchronously with the rotation of the gantry, while the data for the prescribed number of slices is acquired. Such a system generates a single helix from a fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Known cardiac CT scanners utilize xe2x80x9celectro-cardio-gramxe2x80x9d (EKG) signals when acquiring scan data. Typically, a plurality of leads are connected to a patient to measure the EKG signal, which indirectly represents a cardiac cycle. The cardiac cycle includes a period of relaxation and dilation of the heart cavities known as diastole, and a period of contraction of the heart during which blood is ejected from the ventricles known as systole. A typical period of time for one cardiac cycle is slightly less than one second. Thus, a heart goes through a substantial portion of its cycle during one gantry revolution. Motion induced image artifacts result from heart motion.
To suppress the image artifact, some cardiac CT scanners correlate the EKG electrical signals with a plurality of mechanical signals of the heart. The electrical signals and the mechanical signals, however, cannot be precisely correlated for each patient. Therefore, extra views of projection data are acquired based on EKG signals. A radiologist then visually selects a best image from the set of reconstructed images.
Methods and apparatus for reducing motion-induced artifacts in computed tomography (CT) imaging are described. In one aspect, a method for imaging a heart is provided in which the heart is scanned to obtain projection data for a plurality of projection views. A differential projection is determined based on a first and a last projection view. A weighting function is applied to the differential projection to minimize motion artifacts, and an inconsistency index is generated from the differential projection, which is used to identify an image reconstruction location.
In another aspect, a processor in the imaging system is programmed to acquire projection data for a plurality of projection views of the heart. The processor is programmed to determine a differential projection based on a first and a last projection set, apply a weighting function to the differential projection and generate an inconsistency index to determine an image reconstruction location.
In yet another aspect, a computer-readable medium in the imaging system is provided which comprises a plurality of records of projection data. A program residing on the computer-readable medium utilizes a plurality of rules to generate a differential projection based on a first and a last projection view, define a weighting function that is applied to the differential projection to minimize motion induced artifacts, and utilize a plurality of rules to determine an inconsistency index to identify an image reconstruction location.
This method directly measures the mechanics of the heart, rather than an EKG electrical signal. In addition, this method utilizes projection data to select a reconstruction location to minimize motion-induced image artifact. Further, implementing the method does not require that additional hardware be used or replaced.